This invention relates to a micropower charge pump circuit for driving the gate of a field effect transistor.
Charge pump circuits are used to drive the gates of field effect transistors to voltage levels in excess of supply rail voltages. Typically, such circuits have been used to drive MOSFET transistors that switch power to electronic loads. For example, in portable computer applications, MOSFET power transistors are used to switch peripheral devices such as disk drives and displays. In such and other applications, the peripheral device is coupled to the source of the MOSFET switch while the MOSFET's drain is coupled to the supply rail. When a MOSFET switch is coupled in this way (as a so-called "high side" driver), it is desirable to drive the gate of the switch at voltages in excess of the supply rail voltage in order to fully turn on and enhance the switch.
Charge pump circuits used to drive MOSFET switches typically employ oscillators in conjunction with a small number of capacitors to multiply or boost the supply rail voltage to a higher gate voltage. In many applications, the power consumed by this type of circuit is usually dwarfed by the power consumed by the load or the MOSFET switch itself. For example, the operating current of a charge pump circuit might be in the range of a few milliamperes while the switched load current might be on the order of amperes. Thus, the power efficiency of the charge pump circuit itself is generally of minimal concern.
However, there are some battery-powered applications (such as lap-top computer applications) where the power efficiency of the charge pump circuit can become important. In these applications, the power efficiency of the charge pump circuit may be a significant factor in determining battery drain and, hence, battery life before recharging or replacement of the battery becomes necessary.
In view of the foregoing, it would therefore be desirable to provide a power efficient charge pump circuit which can rapidly multiply or boost a supply rail voltage so as to drive the gate of a MOSFET or other switch at a voltage in excess of the supply rail voltage.